In continuous ink jet printing, ink from an ink tank is supplied under pressure to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s). The ink is expelled from the orifices in jets which break up due to surface tension in the ink into droplet streams. Ink jet printing is accomplished with these droplet streams by selectively charging and deflecting some droplets from their normal trajectories. The deflected or undeflected droplets are caught and returned to the ink tank, while the others are allowed to impinge on a printing surface.
Ink is returned from the printhead to the ink tank by holding the ink tank under vacuum. As the ink is drawn through the ink return lines from the catcher or the drop generator, air can also be drawn in as well. This ink-air mixture flows through the ink return lines in a combination of plug and surge flow.
If these fluid lines are ported into the top of the ink tank, the ink returning to the tank can splash and spray down on the ink in the tank. This can generate large amounts of foam in the tank. If the foam build up is too large, some of this foam can be sucked out the vacuum port of the tank. This foam can foul up the vacuum pump or the vacuum control system, if allowed to pass through them. To avoid this problem the prior art has employed three strategies. First, one can try to develop low foaming or non-foaming ink. This typically requires the addition of an anti-foaming agent into the ink. It has been found that such chemicals can have a detrimental effect on the ink-paper interaction, hurting print quality and on runnability of the ink jet system. As a result this is not a desirable option. Second, one can place a foam carry over jar in the vacuum line, between the ink tank and the vacuum pump or vacuum control system. This system is still not trouble free. Under some conditions, the foam can fill not only the ink tank but also the carry over jar. This allows the foam to enter the vacuum pump and control system. Even without filling both the ink tank and carry over jar, the foam can be harmful to the vacuum system. When the bubbles at the surface of the foam burst, they send a fine spray of ink mist or droplets into the air. This fine mist can also be drawn into the vacuum pump, causing it damage. The third strategy to deal with foam has been to reduce the amount of foam which is generated.
Since the foam had been produced by the splashing and spraying of ink onto the surface of the ink, the prior art attempted to port the ink return lines into the ink tank below the surface of the ink in the tank. As the return lines also carry air in, along with the ink, numerous large bubbles are formed as the air enters the tank. As a result a large amount of foam is produced by the this porting option as well.
Another prior art option, has been to port the return lines into the fluid tank above the ink level, but aim the fluid ports directly at the tank walls. The fluid then flows down the tank walls. This greatly reduces the amount of foam produced. While beneficial, this option is not sufficient.
The rapidly moving layer of ink flowing down the tank walls, which results from directing the fluid flow at the walls, enters the bulk fluid in the tank at high velocity. This can cause air to be dragged or entrained down into the bulk of the ink. The entrained air bubbles in the ink can be drawn into the ink pump that is ported out of the bottom of the tank. Such air in the ink pumped to the printhead can adversely affect the operation of the ink jet printer.
An addition failing of this prior art design of directing the returning fluid ports is the result of the non-uniform flow out of these ports. The two phase (ink and air) flow in the return tubes tends to be a combination of plug and surge flow. As a result of the plug and surge flow in the return lines, some foam and a spray of mist can be produced as the ink-air mixture exits the fluid port and splashes into the walls of the tank. This can still produce the vacuum system errors found in the earlier prior art.
It is seen that there exists a need for better means for controlling or eliminating the harmful effects of foam in the ink tank without the need for chemical defoamers and without the problem of air bubbles being entrained into the bulk fluid by the rapid flow of ink down the tank walls.